Construction
: Tunnels

For busy downtown stations,
engineers chose cut-and-cover construction for both tunnels and stations.
This technique kept stations close to the surface, minimizing both the
time needed to enter or exit and the cost of building multiple entrances.
In some cases, the avoid surface disruption, the builders bored earth
tunnels close to the surface instead. North of downtown, where river bottom
gave way to the beginnings of piedmont, they used blasting and enormous
moles to dig rock tunnels. For the Yellow Lines trip
to Virginia, a tunnel was prefabricated on land and carefully sunk in
the Washington Channel.